Cyanobacteria, also known as blue-green algae, are photosynthetic bacteria widespread in marine and freshwater environments. Of particular significance for water quality and human and animal health are those cyanobacteria which produce toxic compounds. Under eutrophic conditions cyanobacteria tend to form large blooms which drastically promote elevated toxin concentrations. Cyanobacterial blooms may flourish and expand in coastal waters, streams, lakes, and in drinking water and recreational reservoirs. The toxins they produce can pose a serious health risk for humans and animals and this problem is internationally relevant since most toxic cyanobacteria have a global distribution.
A diverse range of cyanobacterial genera are well known for the formation of toxic blue-green algal blooms on water surfaces. Saxitoxin (SXT) and its analogues cause the paralytic shellfish poisoning (PSP) syndrome, which afflicts human health and impacts on coastal shellfish economies worldwide. PSP toxins are unique alkaloids, being produced by both prokaryotes and eukaryotes. PSP toxins are among the most potent and pervasive algal toxins and are considered a serious toxicological health-risk that may affect humans, animals and ecosystems worldwide. These toxins block voltage-gated sodium and calcium channels, and prolong the gating of potassium channels preventing the transduction of neuronal signals. It has been estimated that more than 2000 human cases of PSP occur globally every year. Moreover, coastal blooms of producing microorganisms result in millions of dollars of economic damage due to PSP toxin contamination of seafood and the continuous requirement for costly biotoxin monitoring programs. Early warning systems to anticipate paralytic shellfish toxin (PST)-producing algal blooms, such as PCR and ELISA-based screening, are as yet unavailable due to the lack of data on the genetic basis of PST production.
SXT is a tricyclic perhydropurine alkaloid which can be substituted at various positions leading to more than 30 naturally occurring SXT analogues. Although SXT biosynthesis seems complex and unique, organisms from two kingdoms, including certain species of marine dinoflagellates and freshwater cyanobacteria, are capable of producing these toxins, apparently by the same biosynthetic route. In spite of considerable efforts none of the enzymes or genes involved in the biosynthesis and modification of SXT have been previously identified.
The occurrence of the cyanobacterial genus Cylindrospermopsis has been documented on all continents and therefore poses a significant public health threat on a global scale. The major toxin produced by Cylindrospermopsis is cylindrospermopsin (CYR). Besides posing a threat to human health, cylindrospermopsin also causes significant economic losses for farmers due to the poisoning of livestock with cylindrospermopsin-contaminated drinking water. Cylindrospermopsin has hepatotoxic, general cytotoxic and neurotoxic effects and is a potential carcinogen. Its toxicity is due to the inhibition of glutathione and protein synthesis as well as inhibiting cytochrome P450. Six cyanobacterial species have so far been identified to produce cylindrospermopsin; Cylindrospermopsis raciborskii, Aphanizomenon ovalisporum, Aphanizomenon flos-aquae, Umezakia natans, Rhaphdiopsis curvata and Anabaena bergii. Incidents of human poisoning with cylindrospermopsin have only been reported in sub-tropical Australia to date, however C. raciborskii and A. flos-aquae have recently been detected in areas with more temperate climates. The tendency of C. raciborskii to form dense blooms and the invasiveness of the producer organisms gives rise to global concerns for drinking water quality and necessitates the monitoring of drinking water reserves for the presence of cylindrospermopsin producers.
There is a need for rapid and accurate methods detecting cyanobacteria, and in particular those strains which are capable of producing cyanotoxins such as saxitoxin and cylindrospermopsin. Rapid and accurate methods for detecting cyanotoxic organisms are needed for assessing the potential health hazard of cyanobacterial blooms and for the implementation of effective water management strategies to minimize the effects of toxic bloom outbreaks.